With a growing variety of telecommunication terminals and service providers, there is increasing demand for session mobility across networks and technology borders. The term “session mobility” means the transfer of an already existing session from a first device to another device, i.e., involving a terminal change, and/or from a first network access point to another network access point. For example, a network access domain change may comprise the hand-over of an on-going mobile telephone session from a GSM domain to a WLAN domain (GSM=Global System for Mobile Communications; WLAN=Wireless Local Area Network).
Usually, discussions on this topic have a broadly based scope due to the multitude of possible scenarios, i.e., different types of mobility, homogeneous/heterogeneous access networks, converged fixed/mobile networks, single/multi-operator environments, etc. In particular, the prospect of future converged fixed/mobile multi-operator networks such as IMS/TISPAN have fuelled the search for a seamless hand-over technique in a heterogeneous network access environment based on packet switching (IMS=Internet Protocol Multimedia Subsystem; TISPAN=Telecommunications and Internet converged Services and Protocols for Advanced Networks).
Currently discussed solutions use mobility capabilities provided by SIP or mobile IP technology developed for mobile networks (SIP=Session Initiation Protocol; IP=Internet Protocol). However, these approaches have significant handicaps. Mobility mechanisms provided by the SIP community, using a re-invite message to redirect a session, have a major drawback in terms of privacy because the user agent server (=UAS), i.e., the called party, can observe that the moving user agent client (=UAC), i.e., the calling party, is in motion.
Within the SIP framework, when a UAC desires to initiate a session (e.g., audio, video, or a game), it generates an invite request. The invite is a request that is sent to establish a session. This request may be forwarded by proxies, eventually arriving at a UAS that can potentially accept the invitation. Once a session has been established under SIP, the need may arise to communicate changes to the existing SIP session, for instance to add a media stream, to change the codec used for a voice media stream or the updating of the session timer.
Modifying an existing session can also involve changing addresses or ports. For example, when a mobile node such as a notebook computer, PDA or mobile phone moves from one point of attachment (wired or wireless) to another point of attachment, that node may be assigned a new IP address. The communication of changed IP address to the existing SIP session is accomplished by sending a re-invite containing a new media description. This re-invite references the existing dialog so that the other party knows that it has to modify an existing session instead of establishing a new session. An invite request sent within an existing dialog is known as a re-invite. It is at this point that the other party learns that the requesting party is in motion.
The mobile IP technology has even two major disadvantages. First, the session hand-over is rather slow, so there is no seamless session continuity. Second, there are many additional signalling messages, additional packet overhead for IP in IP, and all packets have to be routed through the home network of the provider.
WO 2005/002267 describes a method for ensuring continuity of a communication session when a user equipment hands over from a first communication network to a second cellular communication network comprising the steps of performing an authentication procedure for a packet data session with the second network whilst still being attached to the first network and simultaneously performing a packet data session establishment procedure with the second network whilst still being attached to the first network.
US 2004/0233866 A1 discloses a method of a non-real time software download in an IP-based mobile communication system with heterogeneous access technologies based on the SIP. For download recovery in case of hand-over between a first and a second access network using different access technologies while a software download session is ongoing, the SIP user agent associated with a hand-overing terminal, upon detecting the hand-over, sends, without releasing the ongoing session, a transfer request message to a SIP proxy server that has set up and is currently managing the download session, and that server can start operations necessary in order a new session is set up with the user agent in the new location, under the control of a proxy server entrusted with managing the new session, and download is resumed from said last received packet.